mechanical and gas permeation properties of compatibilized polypropylene–layered silicate...

Mechanical and Gas Permeation Properties ofCompatibilized Polypropylene–LayeredSilicate Nanocomposites

Vikas Mittal

Department of Chemistry and Applied Biosciences, Institute of Chemical and Bioengineering, ETH Zurich,8093 Zurich, Switzerland

Received 5 March 2007; accepted 3 June 2007DOI 10.1002/app.26952Published online 10 October 2007 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Nanocomposites of polypropylene withmontmorillonite modified with dimethyldioctadecylammo-nium ions were prepared and the effect of compatibilizerson the mechanical and permeation properties was investi-gated. Compatibilizers were selected on the basis of theirchemical nature, molecular weight, amount of grafting andlocation of the polar groups. Addition of small amount ofcompatibilizers led to improvements in the basal spacingsof clay platelets indicating enhanced exfoliation. The mod-ulus of the composites increased as compared with thevalues without compatibilizer. The oxygen permeationthrough the composite films either increased or remainedunaffected due to possible interfacial free volume enhance-ment owing to the incompatibility of the surface modifica-tion and the compatibilizer. Increasing the amount of com-

patibilizer also increased correspondingly the extent ofexfoliation. The modulus reached a plateau value afterwhich the increasing compatibilizer led to its decrease. Thegas permeation through the composite films remainedunchanged with increase in the amount of compatibilizerowing to a possible balance between the decrease in per-meation due to path tortuity and exfoliation and increasein permeation due to interfacial incompatibility. Theimproving exfoliation improved the yield and break stressindicating that the absence of tactoids can hinder the pre-mature failure owing to better stress transfer. � 2007 WileyPeriodicals, Inc. J Appl Polym Sci 107: 1350–1361, 2008

Key words: poly(propylene) (PP); layered silicates; surfacemodification; compatibilization; nanocomposites

INTRODUCTION

Polymer nanocomposites have been extensively stud-ied in the recent years owing to the tremendousimprovements in the properties of the polymers atmuch lower filler volume fractions.1 These propertiesincluded thermal stability, heat deflection tempera-ture, flame retardency, barrier and mechanical re-sponse, etc. The organically-modified montmorillonitehas been most commonly used as filler reinforce-ment. The montmorillonite tactoids consisting of alarge number of platy layers held together by elec-trostatic forces are required to be exfoliated into theprimary nanometer thin layers to achieve the opti-mum improvement in properties. The surface modi-fication of the montmorillonite particles by exchangeof the surface inorganic cations with long alkyl

ammonium chains, therefore, is carried out.2,3 Thesurface modification helps to achieve the compatibil-ity between the organic polymer chains by loweringthe interfacial tension thus helping in driving theindividual clay layers apart by the incorporation ofpolymer chains in the intergallery space. The highaspect ratio platelets are responsible for the propertyenhancement. The approach has been successfullyapplied to a large number of polar polymers viz.polyurethanes, epoxies, polydimethylsiloxanes, etc.owing to the easy penetration of the polar clay inter-layers by the polar polymer or prepolymerchains.1,4–10 However, the clay interlayer, which isstill partially polar even after the surface modifica-tion, could not be intercalated when nonpolar poly-mers like polyethylene and polypropylene wereused. At best, only a partial exfoliation has beenreported and thus, the increase in the propertiescould not be considered optimum. Polymers likepolypropylene are one of the most extensively usedpolymers with a wide spectrum of applications. Itsbeneficial properties include high thermal anddimensional stability, low density, better processabil-ity, high water permeation resistance, and resistanceto corrosion. Development of the exfoliated nano-

Correspondence to: V. Mittal, SunChemical, St. Mary CrayTechnical Centre, St. Mary Cray, Kent BR5 3PP, England([email protected]).Contract grant sponsors: Swiss National Science Founda-

tion (SNF), TOP NANO 21.

Journal of Applied Polymer Science, Vol. 107, 1350–1361 (2008)VVC 2007 Wiley Periodicals, Inc.

composites, therefore, can further enhance the rangeof applications to upgrade it from a commodity plas-tic to an engineering plastic.

The interplay of enthalpic and entropic forces gov-erns the nanocomposites synthesis. The absence ofstrong enthalpic interactions between the OMMTand polypropylene chains, therefore, is expected tobring about a limited development. Many approacheshave thus been employed to achieve better distributionand dispersion of the clay layers in polypropylene.Polypropylene-graft-maleic anhydride (PP-g-MA) hasbeen commonly used as amphiphilic compatibilizeror nonionic surfactant to compatibilize both the or-ganic polymer and inorganic filler phases.11–19 Thecompatibilizer is directly added while melt com-pounding the polymer with treated clay. Effect ofmolecular weight of PP-g-MA, amount of PP-g-MAand extent of grafting of MA on filler exfoliation andcomposite properties has been extensively studied.Much better dispersions of OMMT in the compositeshave been reported when amount of PP-g-MA withrespect to OMMT was increased, but an optimumamount of grafting was required to achieve bettermixing as higher grafting can also create immiscibil-ity with the polymer matrix.11–13 Although mechani-cal properties also enhanced with the amount of PP-g-MA but after a certain weight percent of PP-g-MA,a decrease in the properties was also observed.20,21

Also the mechanical properties were observed to bebetter for the PP-g-MA of higher molecular weights,though the lower molecular weight counterparts hadbetter dispersion.17 Another important point to notehere is that although increasing the amount of PP-g-MA in the matrix led to an increase in the d-spacing,but the gains in d-spacing have not been generallyreciprocated with the diffusion of pure PP chainsinside the clay interlayer. Ammonium terminatedpolypropylene has also been used to modify themontmorillonite surfaces and exfoliated compositeswere obtained, although no properties for suchcomposites were reported.22 Although exfoliatedcomposites incorporating only PP-g-MA as matrixhave been reported also, but they do not fulfill thegoal of achieving exfoliated polypropylene nanocom-posites.23–24 Theoretical studies have also indicatedthe importance of Flory Huggins parameters or thesolubility parameters for miscibility between the twophases and the stability of the dispersion. Polypro-pylene composites with OMMT were predicted to beonly intercalated at best. Benefit of amphiphilic com-patibilizer to shift the morphology of the resultingcomposites from unintercalated or weakly interca-lated to almost exfoliated was also suggested.25–29

Tendency of OMMT to exfoliate in the polymermatrix has also been predicted to be dependant onthe length of the surfactant chains ionically attachedto the clay surface for surface modification. Su

et al.30 reported the improvement in mechanicalproperties when composites were prepared usingpolymerically-modified clays containing polystyreneoligomers in the modification. But these approacheswere only partially successful with polypropyleneowing to decreased miscibility at the interface. Apartfrom that, in situ polymerization of propylene byusing Ziegler Natta catalysts immobilized on thesurface was also reported. Although clay exfoliationlevels were improved, but only low molecularweight polymers could be formed thus badly affect-ing their mechanical properties.31,32 The dispersionattained by in situ polymerization is also thermody-namically unstable. Apart from that, melt com-pounding approach with direct addition of compati-bilizer is also preferable from process ease point ofview. By employing the melt compounding approachusing PP-g-MA as compatibilizers, the nanocompo-sites have been observed to attain higher stiffnessthan the pure polymer, but no general trends wereobserved for the other mechanical properties. Elonga-tion and impact strength generally decreased onincreasing the weight fraction of the compatibilizer,whereas the yield strength was much more depend-ant on the process and subsequent morphology.

Although PP-g-MA with different specificationshave been commonly used to achieve the nanocom-posites, the use of block copolymers for such pur-poses have been rarely reported for polypropylenecomposites. Block copolymers have been found toshow better performance that the graft polymersbecause of better defined polar and nonpolarhalves.33 Therefore, it would also be of interest tostudy the behavior of block copolymers with poly-propylene as one half. Chou and coworkers34,35

reported a simple approach for the synthesis of suchcopolymers. Apart from that, the polypropylenecomposites generated so far only for mechanicalapplication point of view, but other properties whichcould also be of importance have been neglected.Polypropylene finds an extensive use in the packag-ing industry, therefore, its gas permeation behaviorapart from the mechanical strength is of prime im-portance. Although the mechanical properties arealways morphology dependant, but the same cannotbe true for the permeation properties.

The goal of the present investigation was to studythe effect of compatibilizers with different architec-ture and amount on the filler exfoliation, mechanicaland gas permeation properties of the OMMT-poly-propylene nanocomposites. PP-g-MA of two differentmolecular weight and grafting were used along witha PP-b-PPG. The surface modification of the filler wasalways kept the same to be dioctadecyldimethylam-monium ions. Properties of the composite were corre-lated with the surfactant concentration and the basalplane spacing of OMMT in the composites.

MECHANICAL AND PERMEATION PROPERTIES OF PP NANOCOMPOSITES 1351

Journal of Applied Polymer Science DOI 10.1002/app

EXPERIMENTAL

Materials

Sodium montmorillonite with a trade name of Cloi-site Na was purchased from Southern Clay (Gon-zales, TX). The cation exchange capacity (CEC)36 ofthe sodium montmorillonite was determined byexchanging its sodium ions with Cu(trien)21. Twodifferent kinds of compatibilizers were used: PP-g-MA1 with a trade name of Admer QF551E was sup-plied by Alcan Packaging (Neuhausen, Switzerland),whereas PP-g-MA2 was procured from Aldrich(Buchs, Switzerland). Dimethyldioctadecylammoniumbromide was supplied by Acros Organics (NewJersey, USA). Polypropylene glycol monobutyl etherwas supplied by Aldrich (Buchs, Switzerland). Poly-propylene used was the homopolymer grade H733-07 from Dow (Dow Plastics, Horgen, Switzerland). Ithas a melt flow index of 7.5 g 10 min21 (2308C at2.16 kg load) and a density of 0.9 g cm23.

Synthesis of PP-b-PPG

To a 100-mL three-neck-round-bottom flask, equippedwith condenser, nitrogen inlet-outlet lines and ther-mometer, finely powdered PP-g-MA2 (3.4 g) andtoluene (20 mL) were placed. The mixture was thentransferred to an oil bath heated at 1008C and stirredovernight to totally dissolve PP-g-MA chains in tolueneto give a yellowish colored homogenous mixture.34,35

Polypropylene glycol monobutyl ether (5 g, in excess)with a molecular weight (Mn) of 1000 was added in oneportion. The temperature was increased to 1108C andthe reaction was maintained at this temperature for 5 h.The mixture was then cooled and filtered. The productwas extracted repeatedly with acetone to remove theexcess ether. Figure 1 details the outline of the process.

Filler surface treatment

To render the aluminosilicate surface organophilic,the exchange of the inorganic cations with organic

ammonium ions was carried out as earlierreported.37 To remove any unattached modifier mol-ecules, the product was again suspended in the hotethyl acetate-ethanol (9 : 1) mixture for few hours at708C, sonicated, filtered, washed, and dried undervacuum at 708C. Both degree of exchange and thepurity of the product were monitored by Hi-ResTGA. All measurements were carried out in air inthe temperature range 50–9008C. If the TGA thermo-gram indicated the presence of unattached modifiermolecules, the washing step was repeated until thepurity of the product was satisfactory. Thus it wasalways ensured that the OMMT is free from anyexcess modifier molecules which if present canadversely affect the properties of composites owingto their low temperature stability.38,39 Finally themodified clay was suspended in 400 mL of dioxane,sonicated and freeze dried. The freeze dried claywas sieved (60 lm mesh) to obtain the fine OMMTpowder.

Composite preparation and characterization

The required amounts of OMMT and polymer werecalculated on the basis of desired inorganic volumefraction as reported earlier using the equation37:

MOMMT ¼ MM þ ðMM 3 CEC 3 MOCÞ

MP¼ðMM 3Vp 3 qP=VM 3 qMÞ�ðMM 3CEC3MOCÞ

where MOMMT is the mass of the OMMT, MM is themass of the inorganic aluminosilicate, MOC is themolar mass of the organic ammonium cation used tomodify the montmorillonite surface, MP is the massof polypropylene, VM is the inorganic volume frac-tion, qM is the density of sodium montmorillonite(2.6 g cm23), VP is the polypropylene volume frac-tion, and qP is its density. Twin blade compounder(Plasticorder W 50 EH, Brabender, Duisburg, Ger-many) equipped with a cavity of 60 cm3 was usedto prepare polypropylene-OMMT composites withrotors operated in counter rotation mode. All thedetails regarding the procedure of compoundingand compression molding to generate thick compo-sites plates and thin films is reported before.37

Wide angle X-ray diffraction (WAXRD) and trans-mission electron microscopy (TEM) was performedon the composite films and oxygen permeation(238C, 0%RH) through the composite films wasmeasured as reported earlier.37

Tensile testing

Dumbbell-shaped samples (type 5B) were stampedout of the compression molded plates using a stamp-ing press (H. W. Wallace, Croydon, Surrey, England)

Figure 1 Reaction strategy to modify PP-g-MA moleculeswith polypropylene glycol monobutyl ether.

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with a die conforming to the standard ISO 527-2.Tensile tests on the stamped samples were carriedout (ISO 527-1) at room temperature using a ZwickZ020 universal testing machine with testXpert 9.01software (Zwick, Ulm, Germany) coupled with aVideo-Extensometer V4.19.02 (Messphysik, Fursten-feld, Austria) for accurate measurement. The drawingspeed of 0.1 mm min21 was used for the measure-ment of elastic modulus and modulus was deter-mined in the range of 0.05%–0.25% strain. A speedof 6 mm min21 was used for the measurement ofother tensile properties and an average of five meas-urements was taken.

Thermal analysis

Differential scanning calorimetric measurementswere carried out on a DSC 7 (Perkin–Elmer, Nor-wark, CT) under nitrogen at a heating rate of 108Cmin21, adopting the procedure for measuring heatcapacities.37,40 High-resolution (Hi-Res) thermogravi-metric analysis (TGA) of OMMT, in which the heat-ing rate is coupled to mass loss, that is, the sampletemperature is not raised until the mass loss at aparticular temperature is completed, was performedon a Q500 thermogravimetric analyzer (TA Instru-ments, New Castle, DE). All measurements were car-ried out in air in the temperature range 50–9008C ata heating rate of 208C min21. Both degree ofexchange and the purity of the OMMT were moni-tored by Hi-Res TGA. TGA of the composites wasalso performed at the same specifications to studythe thermal behavior of the composites as a functionof the increasing amount of OMMT.

RESULTS AND DISCUSSION

To study the effect of nonionic compatibilizers onthe permeation and mechanical performance ofthe OMMT-polypropylene nanocomposites, differentcompatibilizers have been selected on the basis ofchemical composition and the location of the polargroups in the polymer chain. PP-g-MA has beenextensively studied compatibilizer in the literaturefor reducing the interfacial tensions between theOMMT and the polymer. Two different kinds of PP-g-MA have been employed in the present study as is

shown in Table I. PP-g-MA1 has a high molecularweight as indicated by its low MFI and the MA con-tent is low, whereas PP-g-MA2 has a Mn of only3900 (MW 9100) and has 4 wt % of MA content cor-responding to 3.7 MA units per polymer chain, thus,making it a high MA compatibilizer. Acid number ofPP-g-MA2 is 47 mgKOH g21 of PP-g-MA. The den-sities of both compatibilizers are very near to that ofpolypropylene. The limited availability of any blockcopolymers of polypropylene with other polar poly-mers has hindered their use for such applications.To analyze the effect of block copolymers on the per-formance of nanocomposites, PP-b-PPG (polypropyl-ene glycol) was synthesized following the approachreported in literature.34,35 Although the resulting co-polymer is not strictly diblock copolymer, but can berepresented as a polypropylene chains containingpolar blocks as side chains instead of small MA mol-ecules. The properties of PPG chains attached to thePP-g-MA2 have been shown in Table I. The synthesisof the PP-b-PPG was confirmed by IR as shown inFigure 2. The FTIR analysis of PP-g-MA2 showed thecharacteristic absorption at 1852 and 1780 cm21 foranhydride carbonyls and an additional carbonylabsorption at 1710 cm21, revealing the existence offree carboxylic acid [Fig. 2(a)]. The presence of esteradsorption peak of 1734 cm21 and C��O��C polyoxy-alkylene absorption at 1107 cm21 in the FTIR of theproduct [Fig. 2(b)] confirmed the required reaction.The amount of the compatibilizer in the compositeswas also kept to be low in order not to affect thecrystallization behavior of the composites. A compa-tibilizer/OMMT weight ratio of 0.16 was used in thecomposites. Cloisite was used as the inorganic mont-morillonite owing to its relatively high cationexchange capacity of 880 l equiv g21 which leads toa dense packing on the clay surface when the inor-ganic cations present on the surface are ionexchanged with long chain alkyl ammonium ions.Dimethyldioctadecylammonium ions were exchangedon the surface because of their similar chemical na-ture of polypropylene chains. As mentioned earlier,the clay was carefully exchanged to have fullexchange on the surface and to avoid the presenceof any excess surfactant molecules because of thedetrimental effect of these excess molecules on theproperties of the composites owing to their low

TABLE IDetails of the Compatibilizers/Grafted Oligomers used in the Study

(Supplier Information)

Property PP-g-MA1 PP-g-MA2 PPG monobutyl ether

Mn (g/mol)/MFI 5.6 g (10 min, 2308C, 2.16 kg) 3900 1000Density (g/cm3) 0.89 0.934 0.989Viscosity 2.3 P at 1908C 4 P at 1908C 140 cSt at 208CMA content (wt %) 0.1 4 –



temperature stability. Apart from that, the com-pounding temperatures were also kept not high soas to degrade the ammonium ions, but high enoughfor efficient mixing.

The 001 basal plane spacing values of the compo-sites with different compatibilizers have beenreported along with the composite without any com-patibilizer in Table II. Figure 3 contains the WAXRDdiffractograms of composites with and without com-patibilizer for comparison. The basal plane valueswere augmented marginally on the addition of com-patibilizers. The increase in the d-spacing was how-ever observed to be proportional with the polarity ofthe compatibilizer molecules. One thing to be notedhere is that the presence of diffraction peaks in everycomposite indicates that the clay was never fullyexfoliated in any composite. The clay without anycompatibilizer was in fact also marginally interca-lated as is evident from an increase in the basal

plane spacing from 2.56 to 2.82. Because of its highmolecular weight and very low grafting percent ofMA, PP-g-MA1 can be expected to be least helpfulin improving the d-spacing. PP-g-MA2 also showedalmost the same d-spacing as the composite withoutthe compatibilizer. The basal plane spacing ofOMMT in composite with PP-b-PPG as compatibil-izer was highest owing to its substantially polar na-ture. Since the intensity of the reflections in the dif-fractogram is dependant on the orientation of thetactoids, thus, it cannot be used to compare witheach other to get a quantitative idea of exfoliatedtactoids.

The gas permeation of the composite without theaddition of compatibilizers was reduced by around35% indicating that a partial exfoliation still couldtake place even without the addition of the compati-bilizer. The addition of the compatibilizers wasexpected to bring about a decrease in the permeationthrough the composites films, but opposite behaviorswere observed. Permeation through the compositefilms was observed to increase in PP-g-MA1 and PP-

Figure 2 FTIR spectrum of (a) PP-g-MA2 and (b) theproduct obtained after the reaction of PP-g-MA2 withpolypropylene glycol monobutyl ether.

TABLE IId-Spacing and Oxygen Permeability of 3 vol % 2C18

OMMT-Polypropylene Nanocomposites usingDifferent Compatibilizers

Compatibilizer(2 wt %)

d-spacing(nm)

Permeabilitycoefficienta

(cm3 lm/m2 day mmHg)

Pure PP – 89No compatibilizer 2.82 59PP-g-MA1 2.68 84PP-g-MA2 2.89 63PP-b-PPG 3.09 79

a In the range of 5% error.

Figure 3 X-ray diffractograms of 2C18 OMMT and its 3vol % nanocomposites with and without the addition on 2wt % compatibilizer.

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b-PPG, whereas it was almost unchanged in PP-g-MA2. Although a little better exfoliation of the plate-lets can definitely be expected from PP-g-MA2 andPP-b-PPG as indicated by the improvements in d-spacing, the permeation results do not seem to corre-late with these gains in d-spacing. Although it iscommonly suggested, that the properties of the poly-mer composites are dependant on morphology and,therefore, more exfoliation should bring furtherenhancements in all the properties. But this observa-tion is more reliable and relatable to the bulk me-chanical properties rather than the gas permeationbehavior because of the sensitivity of these microproperties to many other factors also. One prime rea-son is the compatibility of the treated clay with thecompatibilizer molecules. Although these moleculescan adsorb on the surface and increase the clay inter-layers by intercalation, but their compatibility withthe surface modification can play a very decisiverole on the permeation properties. Microvoids orincrease in free volume generated at the interfacebecause of the incompatibility of these two phasescan lead to an increase in the permeation, thoughthe clay is more exfoliated thus counterbalancing theeffect of exfoliation leading to no decrease orincrease in the permeation. Similar behavior havealso been observed earlier in polyurethane and ep-oxy composites in which OMMT modified with dio-ctadecyldimethyammonium ions led to an increasein oxygen permeation owing to the incompatibilityof polar polymer with the 2C18 chains, whereas thisbehavior was absent when 2C18 was replaced by po-lar surface modifications.6,41 Thus, permeation apartfrom morphology is also dependant on the interac-tion of the functionalized polymer (i.e., compatibil-izer) and the surface modification. The increase inthe permeation through the composite containingPP-b-PPG as compatibilizer can, therefore, beexplained as due to the incompatibility between thecompatibilizer and surface modification. Too highpolarity of PP-b-PPG can also be held responsible forits incompatibility with the polymer matrix. PP-g-

MA1 although least polar has maximum increase inthe oxygen permeation. Owing to its very high mo-lecular weight and the presence of very few MAmolecules present per chain, the entanglement ofthese long chains can be visualized to increase thefree volume at the interface with clay surface, lead-ing to much higher increase in permeation. PP-g-MA2 on the other hand, owing to its low molecularweight can well mix with the polymer and canadsorb on the surface and enter the clay interlayerseasily, thus improving their exfoliation. Thus, in thiscase, a balance of two factors i.e., increase in perme-ation because of increase in voids at the surface anddecrease in permeant mobility and increase in tortu-ous path owing to exfoliation seem to occur. Verydifferent results were observed in polyethylene com-posites using PE-g-MA as compatibilizer indicatingthe sensitivity of the permeation behaviors to thesystem changes.33

It was established in the earlier study that thepresence of the OMMT did not lead to any substan-tial changes in the crystallization behavior of poly-propylene.37 Thus the change in the properties wassolely related to the effect of tortuosity generated bythe platelet exfoliation and interfacial interactions.Although the amount of the nonionic surfactant waskept low in the present study in order not to affectthe crystallinity of the polymer, however, calorimet-ric studies were necessary to monitor the effect ofcompatibilizer addition. As is evident from Table III,no changes in melting and crystallization behavior ofpolymer could be observed when the different sur-factants were added. Percent crystallinity values cal-culated from the enthalpy of melting of the polymerin the composites and the enthalpy of melting ofpurely crystalline polypropylene sample were alsoobserved to be unchanged. It confirms that thesurfactants did not induce any changes in the calori-metric behavior of the polymer. To supplement theX-ray findings, the microstructure of the compositeswas also studied by transmission electron micros-copy. Figure 4 shows the TEM micrographs of 2C18

TABLE IIICalorimetric Behavior of 3 vol % 2C18 OMMT-Polypropylene Nanocomposites using Different Compatibilizers


Tm,onseta

(8C)Tm

b

(8C)DHm polymer

(J/g)Crystallinityc

(Xc)Tc,onset

d

(8C)Tc

e

(8C)

Pure PP 152 162 96 0.58 114 112No compatibilizer 152 163 94 0.57 115 112PP-g-MA1 153 162 96 0.58 120 114PP-g-MA2 152 163 94 0.57 115 110PP-b-PPG 151 163 95 0.57 115 109

a Onset melting temperature.b Melt peak temperature.c Degree of crystallinity calculated using DH of 100% crystalline PP 5 165 J/g (Refs. 42, 43).d Onset crystallization temperature.e Peak crystallization temperature.



OMMT-PP nanocomposites with 2 wt % of PP-g-MA2. A number of well exfoliated single layers areclearly visible indicating the higher extents of exfoli-

ation. But thin tactoids are also seen which furtherindicates the presence of mixed morphology contain-ing thin tactoids and exfoliated layers. Another im-

TABLE IVTensile Properties of 3 vol % 2C18 OMMT-Polypropylene Nanocomposites using

Different Compatibilizers


Tensilemodulusa (MPa)

Yield stressb

MPaYield

strainc (%)Stress at

breakd (MPa)

pure PP 1510 36 9.4 29No compatibilizer 2016 28 4.3 25PP-g-MA1 2131 28 5.3 26PP-g-MA2 2124 28 4.4 23PP-b-PPG 2088 27 3.3 26

a Relative probable error 5%.b Relative probable error 2%.c Relative probable error 5%.d Relative probable error 15%.

Figure 4 SEM micrographs of 3 vol % 2C18 OMMT-polypropylene nanocomposites containing 2 wt % of PP-g-MA2 com-patibilizer.

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portant observation was the misalignment, foldingand bending of the clay platelets and tactoids. Low-ering the magnification did seem to show a globalorientation of the tactoids but at the higher magnifi-cations, this orientation seem to be absent.

Table IV contains the tensile properties of theOMMT-polypropylene composites with and withoutthe compatibilizers consisting of 3 vol % of inorganicfiller fraction. The properties of theses compositesare compared with the pure polypropylene proc-essed similarly as the composite materials. The com-posite without compatibilizer was observed to havean improvement in the modulus owing to the partialexfoliation of the OMMT. All other properties wereobserved to decrease indicating lack of attractiveintercations at interface and the presence of brittle-ness. Tensile modulus was enhanced on the additionof compatibilizers indicating increased exfoliation ofthe OMMT resulting in better morphology andhence modulus. One thing to notice here is the dete-rioration of the permeation behavior happened par-

allel to improvement in mechanical performance.Mechanical properties owing to their macro natureseem to remain independent or less affected bymicro voids generated owing to incompatibility ofthe surface modification and the compatibilizer andthus more and more load could still be transferredto the clay platelets. Figure 5(a) also shows the corre-lation between the relative modulus and the d-spac-ing of OMMT in these composites. The modulus wasobserved to be roughly unchanged with respect tod-spacing indicating only exfoliated layers are re-sponsible for the improvement in modulus. Also theplasticization effects generally associated with thecompatibilizers may also limit the further improve-ment of the modulus. Yield stress of the compositeswas observed to remain unchanged as comparedwith the composite without compatibilizer indicatingthe presence of little brittleness and nonoptimuminterfacial interactions. Yield strain and the breakingstress of the compatibilized composites had occa-sional marginal increase as compared with theuncompatibilized composite indicating the weakplasticizing effect of the compatibilizers (Table III).Relative break stress was also observed to be inde-pendent of d-spacing, while the yield stress andstrain were observed to decrease with increase in d-spacing [Fig. 5(a,b)]. The presence of tactoids whichare not efficient enough for load transfer can be areason for this decrease.

As PP-g-MA2 had superior improvement in themechanical performance and the permeation throughits composites was better than other surfactant con-taining composites, it was of interest to analyze thesebehaviors with resect to increasing concentration ofPP-g-MA2. Figure 6 shows the WAXRD diffracto-

Figure 5 (a) Tensile and break stress and (b) yield stressand strain of 3 vol % OMMT-polypropylene nanocompo-sites with and without compatibilizer.

Figure 6 X-ray diffractograms of 3 vol % OMMT-poly-propylene nanocomposites with varying amounts of PP-g-MA2 compatibilizer.



grams of the composites with 2, 4, 6, and 8 wt % ofPP-g-MA2 compatibilizer. As can be seen, that the d-spacing was enhanced on increasing the content ofcompatibilizer and the peak width also seemed toincrease indicating qualitatively more and more sep-aration and loss of order among the tactoids. How-ever, the presence of 001 peak even at 8 wt % PP-g-MA2 content also indicates that not all of the clayplatelets have been exfoliated. The basal plane spac-ing values have been plotted against the PP-g-MA2content in Figure 7. The d-spacing did not increaselinearly with the compatibilizer content. It seemsthat there was present a threshold value of d-spac-ing, after which the weakly held platelets were inter-calated significantly even on the addition of sameamount of compatibilizer. The oxygen permeationcoefficients of the composites with different concen-trations of compatibilizers have been reported inTable V. The oxygen permeation coefficients of com-posite without compatibilizer, pure PP and a blend

of PP and PP-g-MA2 (2 wt %) have also been com-pared. The oxygen permeation through the blend ofPP and PP-g-MA2 remained almost unchanged asthe pure PP indicating that at this concentration ofcompatibilizer, it was compatible with PP and didnot affect its crystallization characteristics. On theother hand, as already reported that the oxygen per-meation decreased by roughly 35% when no compa-tibilizer was added indicating that the interactionsbetween the OMMT and the compatibilizer are theresponsible forces for the permeation deteriorationobserved on the addition of compatibilizer. This alsosupports the suspicion of generation of voids at theinterface on the addition of compatibilizer. The per-meation coefficient of the composites with differentamounts of compatibilizer remained unchanged asthe composite without compatibilizer (Fig. 8). It canonly be possible if the compatibilizer did not interca-late inside the interlayer and remained outside inthe matrix and thus did not affect the microstruc-ture. But the increase in the basal plane spacing andthe improvement in the mechanical performance ear-lier observed indicate that the microstructure wasdefinitely altered from less exfoliated to more andmore exfoliated state with increasing the amount ofcompatibilizer. This can only be justified by the ear-lier explained balance of improved permeation per-formance due to tortuosity generated and exfoliationof the platelets versus the deteriorated permeationbehavior due to increase in the number of voids dueto more and more exfoliation. Calorimetric studieswere also performed on the composites containingdifferent amounts of compatibilizer as shown inTable VI to see the effect of compatibilizer. The melt-ing and the crystallinity behavior were observed to

Figure 7 Enhancement of the basal plane spacing withincreasing PP-g-MA2 weight fraction in the composites.

TABLE VEffect of PP-g-MA2 Weight Fraction on d-spacing and

Oxygen Permeability of 3 vol % 2C18 OMMT-Polypropylene Nanocomposites

PP-g-MA2(wt %)

d-spacing(nm)

Permeabilitycoefficienta

(cm3 lm/m2 day mmHg)

Pure PP – 892.0, no filler – 830.0 2.82 592.0 2.89 634.0 2.90 626.0 3.12 628.0 3.15 61

a In the range of 5% error.

Figure 8 Relative oxygen permeability through the com-posite films containing 3 vol % of 2C18 OMMT and vary-ing weight fractions of PP-g-MA2.

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be unchanged and the degree of crystallinity wasalso found to be slightly decreasing with increasingthe amount of the compatibilizer. This indicates thatthe thermal behavior of the polymer remained over-all unchanged even at 8% addition of PP-g-MA2.

The tensile properties of the composites with vary-ing amounts of compatibilizer were measured andare shown in Table VII along with pure PP, blend ofPP and PP-g-MA2 and OMMT-PP composite withoutcompatibilizer. The modulus of the PP and PP-g-MA2 blend was marginally decreased as comparedwith that of pure PP indicating the plasticizationeffect of the compatibilizer. As a result, the yieldstrain was also observed to increase. It should alsobe noted that no such effect was observed whensame amount of compatibilizer was added in thepresence of the OMMT. The relative values havealso been plotted with respect to the compatibilizerweight fraction in Figure 9. The tensile modulus ini-tially increased but then decreased with augmentingthe amount of compatibilizer. As reported earlier,33

the improvement in the mechanical properties arejointly affected by the plasticization and exfoliation.At low weight fraction of the compatibilizer, the

exfoliation effect is more significant, but become lesspronounced when the amount of compatibilizer isincreased. Thus the presence of low molecularweight chains in large amount hinder the process ofstress transfer thus reducing the modulus. Stress atbreak was observed to increase whereas yield stressremained unchanged for the composites withincreasing the amount of compatibilizer due to simi-lar plasticization effect. The yield strain decreaseslightly indicating further brittleness. It is also im-portant to note that the break stress and the yieldstress are generally observed to decrease on increas-ing the filler volume fraction in the absence of com-patibilizer. This clearly indicates that the presence oftactoids in the system can lead to early failure,whereas the system situation improves as theamount of this tactoids is reduced.

Thermogravimetric studies were also carried outon the composites to analyze the effect of theincreasing amount of compatibilizer on the thermalstability of the composites. Figure 10 shows the TGAthermograms of the composites (represented as bcontaining 0, 2, 4, 6, 8 wt % compatibilizer). Alsoshown are the TGA curves for pure polypropylene

TABLE VIEffect of PP-g-MA2 Weight Fraction on the Calorimetric Behavior of 3 vol % 2C18

OMMT-Polypropylene Nanocomposites

PP-g-MA2 (wt %) Tm,onseta (8C) Tm

b (8C) DHm polymer (J/g) Crystallinityc (Xc) Tc,onsetd (8C) Tc

e (8C)

Pure PP 152 162 96 0.58 114 1122.0, no filler 152 165 94 0.57 112 1060.0 152 163 94 0.57 115 1122.0 152 163 94 0.57 115 1104.0 152 163 93 0.56 115 1106.0 152 163 93 0.56 115 1108.0 152 160 93 0.56 114 109

a Onset melting temperature.b Melt peak temperature.c Degree of crystallinity calculated using DH of 100% crystalline PP 5 165 J/g (Refs. 42, 43).d Onset crystallization temperature.e Peak crystallization temperature.

TABLE VIIEffect of PP-g-MA2 Weight Fraction on the Tensile Properties of 3 vol % 2C18

OMMT-Polypropylene Nanocomposites

PP-g-MA2(wt %)

Tensilemodulusa (MPa)

Yieldstressb (MPa)

Yieldstrainc (%)

Stress atbreakd (MPa)

Pure PP 1510 36 9.4 292.0, no filler 1463 33 10.5 360.0 2016 28 4.3 252.0 2124 28 4.4 234.0 2075 28 3.9 276.0 1984 27 3.3 278.0 1913 28 3.3 27

a Relative probable error 5%.b Relative probable error 2%.c Relative probable error 5%.d Relative probable error 15%.



and the blend of PP and PP-g-MA2 (2 wt %) repre-sented as a in the figure. It is clear from the thermo-grams that the presence of compatibilizer in thecomposites did not lead to any unwanted prematurethermal degradation of the composites irrespectiveof the low molecular weight of the compatibilizers.Thus, the presence of OMMT is much more signifi-cant in improving the thermal behavior and is notaffected by the presence of low molecular weightcompatibilizer.

CONCLUSIONS

Addition of compatibilizers help to exfoliate the clayplatelets by bringing down the interfacial surfaceenergy. The more the polar compatibilizer is, themore improvement in the basal plane spacing can beobserved. The exfoliation achieved help to improvethe mechanical performance of the composites. How-ever, the gas barrier properties are not similarlyenhanced owing to suspected negative effect fromthe incompatibility of the surface modification with the compatibilizers used leading to micro voids at

the interface. The montmorillonite platelets are com-pletely misaligned, bent and folded indicating a pos-sible loss in the improvement in the properties dueto misalignment. Increasing the amount of PP-g-MA2 further enhances the exfoliation, but the tensilemodulus shows a decrease after reaching a maxi-mum value owing to the low molecular weight ofthe compatibilizer. The permeation through the com-posite films is not affected with the concentration ofthe compatibilizer. The break stress increases whilethe yield stress is observed to be constant withrespect to the compatibilizer concentration indicatingthe plasticization effect of the compatibilizer. Theyield strain decreases with compatibilizer contentindicating the brittleness of the system. The additionof even 8 wt % of the compatibilizer does not bringabout any change in the crystallization behavior ofpolypropylene. Thermal behavior of the compositesalso remains unaffected.

The author thanks Prof. M. Morbidelli and Prof. U.W.Suter for allowing the use of their research facilities.Expert help of Dr. N. Matsko for TEM investigations ishighly appreciated.

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